Collaborative Research: Helium-isotope Systematics Along Seismic Profiles in Tibet to Study Geometry of Indian and Tibetan Lithosphere

喜马拉雅-西藏是印度和亚洲之间地球上活跃的大陆-大陆碰撞的典型例子，但板块尺度的几何形状仍然存在争议。来自斯坦福大学和新墨西哥大学的研究人员将对喜马拉雅和西藏温泉的流体进行采样和分析，以确定印度岩石圈的北端(和亚洲岩石圈的南端)，从而确定印度被迫(或被俯冲)到亚洲以下的北端。这一努力将提高科学家绘制喜马拉雅山和西藏的准确横断面的能力，使其达到比以前可能的更深。这种对地球的新认识？S最戏剧性的山脉带和高原提高了公众对科学的兴趣，并为将地球科学纳入高中课程提供了一个自然的机会。为此，将制作一段关于研究项目和相关课程材料的新视频，以满足下一代科学标准。大陆碰撞的岩石圈几何仍然存在争议。关于印度地壳和地幔在西藏下方向北穿透一定距离的单一造山剖面，目前还没有达成一致意见。相反，越来越多的地震学观点支持从西向东、从地壳俯冲到岩石圈拆沉(俯冲回滚)再到去斑点的巨大变化的模型。斯坦福大学帮助获取和解释了喜马拉雅/西藏多个地震断面的数据。这些和其他已发表的剖面对深层反射器和转换器的解释为印度和西藏岩石圈的几何形状提供了可测试的模型，因为整个西藏的岩石圈从西向东变化。温泉中3He/4He比值的初步测量表明，中国西藏(西藏和青海)拉萨地块的地幔脱气作用显著，而且在空间上是可变的。研究小组将在西藏多达60个新地点进行地热和碳酸盐流体以及相关导线的采样，在两个野外季节期间，将沿着几个横断面进行采样，这些横断面是根据其可访问性、地震数据的可用性和构造/地质环境而选择的。该团队将分析3He/4He、22Ne、36Ar、Delta13C、Delta18O、DeltaD、87Sr/86Sr、主量元素和微量元素，以约束流体来源(地幔、地壳、大气、沉积、有机)，了解流体混合和路径，并使用多组分化学地质测温法计算储层温度。将重新分析地震数据，包括宽带远震和近垂直反射数据，以更好地了解采样流体的源区和传输路径。这些数据将限制印度克拉通地幔岩石圈和西藏早期熔融地幔之间的边界，并有助于确定西藏下方的地幔缝合。

Himalaya-Tibet is Earth's type example of active continent-continent collision, between India and Asia, yet the plate-scale geometry remains controversial. Researchers from Stanford University and the University of New Mexico will sample and analyze fluids from hot-springs, which are tracers of deeper structure, in the Himalaya and Tibet to locate the northern limit of the Indian lithosphere (and southern limit of the Asian lithosphere), hence the northern limit to which India has been forced (or underthrust) beneath Asia. This effort will improve scientists' ability to draw accurate cross-sections through the Himalaya and Tibet to greater depths than previously possible. This new understanding of Earth?s most dramatic mountain belt and plateau enhances public interest in science, and offers a natural opportunity to integrate earth science into high-school curricula. To this end, a new video about the research project and associated curricular materials designed to meet the Next Generation science Standards will be produced.The lithospheric geometry of continental collision remains controversial. There is no agreement on a single orogenic cross-section in which Indian crust and mantle penetrate a certain distance northwards beneath Tibet. Instead, growing seismological opinion supports models with large changes from west-to-east, from crustal underthrusting to lithospheric delamination (subduction roll-back) to de-blobbing. Stanford has helped acquire and interpret data from multiple seismic transects across Himalaya/Tibet. The interpretations of deep reflectors and converters from these and other published profiles provide testable models for the geometry of Indian and Tibetan lithospheres as it varies from west to east across Tibet. Pilot measurements of 3He/4He ratios in hot springs show significant, and spatially variable, mantle degassing across the Lhasa block in Chinese Tibet (Xizang and Qinghai). The research team will sample geothermal and carbonic fluids and associated travertines at up to 60 new locations in Tibet during two field seasons along several transects chosen for their accessibility, availability of seismic data, and tectonic/geologic setting. The team will analyze 3He/4He, 22Ne, 36Ar, delta13C, delta18O, deltaD, 87Sr/86Sr, major and trace elements to constrain fluid sources (mantle, crustal, atmospheric, sedimentary, organic), to understand fluid mixing and pathways, and to calculate reservoir temperatures using multi-component chemical geothermometry. Seismic data, both broadband teleseismic and near-vertical reflection data, will be re-analyzed to better understand the source regions and transport pathways of the sampled fluids. The data will constrain the boundary between Indian cratonic mantle lithosphere and Tibetan incipiently melting mantle and help define the mantle suture beneath Tibet.